HR: 16:00h
AN: NS14A-03    [Abstracts]
TI: Downhole Measurement of Shear-Wave Splitting in Holocene Sediments: Application to Landslide Hazards in the Ottawa Valley, Canada
AU: * Harris, J B
EM: harrijb@millsaps.edu
AF: Millsaps College, Department of Geology, 1701 N. State St., Jackson, MS 39210 United States
AU: Hunter, J A
EM: JHunter@NRCan.gc.ca
AF: Geological Survey of Canada, 601 Booth St., Ottawa, ON K1A 0E8 Canada
AU: Aylsworth, J M
EM: JAylswor@NRCan.gc.ca
AF: Geological Survey of Canada, 601 Booth St., Ottawa, ON K1A 0E8 Canada
AU: Burns, R A
EM: burns@NRCan.gc.ca
AF: Geological Survey of Canada, 601 Booth St., Ottawa, ON K1A 0E8 Canada
AU: Good, R L
EM: RGood@NRCan.gc.ca
AF: Geological Survey of Canada, 601 Booth St., Ottawa, ON K1A 0E8 Canada
AB: Stress-aligned inclusions (pore spaces and microfractures) cause seismic shear waves to exhibit directional polarizations in response to propagation through azimuthally anisotropic sediments. This phenomenon, called shear-wave splitting, is observable by measuring differences in shear-wave velocity between waves traveling parallel (S1 - fast shear wave) and perpendicular (S2 - slow shear wave) to the trend of the inclusions. This investigation provides evidence of shear-wave splitting in Holocene sediments of the Ottawa Valley (southern Ontario), Canada. Preliminary analysis of a multicomponent downhole data set, recorded using hammer and mass energy sources, indicates shear-wave splitting related to in-situ stresses associated with steep, landslide-prone, slopes bounding the South Nation River. Orientation of the split shear waves, determined from particle motion plots of the three-component downhole records, shows alignment of the polarization directions with the local topography and analysis of travel-time delays between S1 and S2 suggests anisotropy values of approximately 7% for the upper 30 m of the subsurface. Temporal monitoring of changes in shear-wave splitting may provide a method to evaluate factors related to landsliding, and assist in identifying potentially unstable slopes.
DE: 0915 Downhole methods
DE: 0935 Seismic methods (3025)
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly